The present invention relates to video signal skew, and in particular to devices and methods for determining and compensating for skew between video signals transmitted over wires.
When video colour signals are separately transmitted over wires of different lengths, then the signals become temporally separated (skewed) which degrades the image generated by a video display device. This can particularly be a problem in a class of devices known as KVM (Keyboard, Video monitor and Mouse) extenders. These devices allow a KVM console to be used to control a distant computer over distances of several hundred metres.
Such devices often use twisted pair cabling to transfer video signals from the computer to the monitor, and data and control signals between the computer and mouse and keyboard. However, twisted pair cables are more suited to carrying high speed digital signals, rather than analogue video signals. Different pairs of cables have different twist rates, i.e. number of twists per unit length, in order to reduce crosstalk between the pairs and improve their high speed digital performance.
The different twist rates means that different pairs extending over the same distance will actually have different signal path lengths. Hence different colour signals transmitted over different pairs of cables will be delayed compared to signals transmitted over the same distance but having a shorter signal path length. This introduces a timing skew between the colour signals resulting in the colours appearing to be ‘split’ when recombined and displayed on a monitor. For example, a white colour block on a black background will appear to have a blue leading edge and a red trailing edge if the green signal is delayed with respect to the blue signal and the red signal is delayed with respect to both the blue and green signals.
Small amounts of colour split can be acceptable to a user. However, colour split problems are more noticeable at higher screen resolutions and refresh rates as the scan time for each horizontal line is reduced and so the skew delay becomes a larger proportion of the horizontal scan time. Also the colour split problem increases with the length of cable. Further, different cable manufacturers provide cables with different twist rates. The cables used to connect a KVM extender are often two or more cables connected together, e.g. by a patch panel, and so it is difficult to known with any accuracy the signal path differences that may arise in practice. Furthermore, newer cable types such as category 5e and category 6 cables tend to have higher twist rate differences exacerbating video signal skews.
Colour split is not always a problem, for instance over short distances and/or using lower resolution monitors, therefore it is not always necessary to provide some means to compensate for the differences in the video signal path lengths. However, a cheap and simple way of determining quantitatively when skew compensation may be required and the amount of skew compensation required would clearly be advantageous.
A previous method of providing skew compensation is the use of delay lines constructed from electronically and structurally complex capacitor and inductor networks. However, such networks that provide the high bandwidths required for video monitor colour signals are very expensive. Cheaper versions using less effective components tend to have more limited bandwidth and tend to result in an unacceptably blurred video image.
An alternative approach would be to use sections of twisted pair cables of various lengths. However, a skew compensation device constructed in this way would be bulky, and difficult and untidy to construct. Therefore a cheap and tidy skew compensation device would be advantageous.